This application claims the priority of Austrian Patent Application, Serial No. 1619/99, filed Sep. 22, 1999, the subject matter of which is incorporated herein by reference.
The present invention relates to a process of making a metal matrix composite (MMC) composite comprised of a preform having a ceramic, porous consistence with pores filled with matrix metal.
MMC is a material in which a preform and a metal are embedded within one another at different quantitative ratios. In the following description, the term xe2x80x9cpreformxe2x80x9d will denote a porous structure made from a reinforcing material. The configuration of the preform material may be selected in a wide variety. For example, the preform may be comprised of single parts of random geometry (spherical, angular, of short fibers or long fibers, or in form of whiskers). It is also possible to provide the preform of one or more bodies of different sizes and porous structure.
Metal is poured to infiltrate the spaces between the individual parts and/or in the pores of a porous body. The so-called infiltration process is substantially a casting process by which metal is introduced into the very narrow pores of the preform. In case of a poor wetting behavior between the material of the preform and the metal in the pores, the metal must be forced into the voids of the pores. This is typically implemented through pressure application by which pressure, e.g. gas pressure or mechanical pressure by means of a die, is applied upon the liquid metal for pressing the metal into the pores of the preform. Another example of an infiltration process includes the decrease of surface tension of the liquid metal to such an extent that the metal can penetrate the pores of the preform even without application of a particular pressure. This type of infiltration process is commonly referred to as xe2x80x9cspontaneous infiltrationxe2x80x9d. The decrease of the surface tension of the metal can be realized by reacting the liquid metal with respective agents that decrease the surface tension, for example by mixing these agents to the liquid metal or by introducing these agents into the ambient atmosphere of the liquid metal.
The solidification of the liquid metal is realized in the event of non-wetted preform-metal systems under pressure to prevent a segregation, i.e. to prevent metal introduced into the preform from oozing out from the pores again.
The purpose of the preform is to impart the finished composite and/or the used matrix material with particular or special mechanical, thermal and electrical properties, which are substantially determined by the arrangement and composition of the preform. For example, the volume portion together with the modulus of elasticity governs the expansion coefficient. A high modulus of elasticity and a high packing density result in a low expansion coefficient whereas a low modulus of elasticity and a same packing density leads to a comparably high expansion or requires a very high volume portion. Different compositions of the material influence, for example, the thermal conductivity. A composite of aluminum and Al2O3 has a substantially lower conductivity than a composite of Al and SiC. Moreover, the type of infiltration metal impacts on the mechanical, electrical and thermal properties of the resultant MMC. Examples for typical infiltration metal includes aluminum, magnesium, copper as well as alloys comprised of one or more of these metals. Of course, other metals or metal-like elements may be usable as well.
The production of porous ceramic components that can be used as preform for such MMC components includes a variety of shaping processes, such as injection molding, hot casting, dry pressing, film casting, vacuum casting and slip casting. Each of the mentioned processes uses ceramic powder with a binder, lubricant etc., and the mixture is made flowable and shaped into the desired configuration. Slip casting may be carried out without addition of a binding agent.
By way of example only, the following description refers in more detail to the hot casting process. In this process, thermoplastics (up to 45% by volume), such as, e.g., paraffins or waxes, are used as binders and added to the ceramic powder. By raising the temperature, this mixture becomes liquid, and the hereby obtained flowing mass is poured into a mold. After solidification of the binder, the preform is ejected, the binder is expelled through heating, and the material is sintered. Depending on the duration of the sintering process and the selected sintering temperature, a porous, self-contained or dense structure is produced.
Preforms may also be produced by an injection molding process in which a powder mixture blended with binder (up to 40% binder fraction is required) is injected into a respective mold and allowed to cure. The binder is removed thereafter in a same manner as described with reference to the hot casting process.
A further possibility is the dry pressing process of ceramics, in which a flowable powder mixture is filled into a respective preform mold and pressed by a die into the desired shape. The required binder or also assisting pressing agents may hereby be provided of less complex configuration compared to the afore-mentioned hot casting process, and the quantity being used is also substantially smaller. Suitable binding agents include stearates and paraffins.
The use of a binder, e.g. stearic acid as oftentimes employed in conventional processes, has the drawback that the extraction of the binder requires a time-consuming processxe2x80x94typically heating of the ceramic particle mass to a temperature above the evaporation point of the binder, after removal of a liquid carrierxe2x80x94and entails the risks that the involved high temperatures result in a deformation of the molded product, and a reaction of gases of the ambient atmosphere, in particular oxygen with the ceramic particles, leading to undesired compounds that adversely affect the properties of the resultant preform and ultimately of the resultant MMC component.
In all these shaping process or preform processes, the required amounts of organic additives are removed again after obtaining the desired configuration, typically through thermal processes. As a consequence of the high fraction of organic substance, this process must be carried out at a slow speed, in particular when the formed body has an irregular geometry because, otherwise, the surface may crack, or deformations in the formed component may be experienced, rendering the use unsuitable. Depending on the particular addition of binder and lubricant, their removal is carried out at temperatures of 300xc2x0 C. to 700xc2x0 C. according to a time schedule that is selected to the specific material and product. Kilns with complicated temperature programs and temperature profiles, which must be maintained very accurately, are required hereby. The process may take several days, depending on the complexity of the component. Also, the subsequent sintering process requires a very slowly and careful heating profile to prevent internal tensions as best as possible. The same care is required for the subsequent cooling to room temperature.
Practice has shown that slip casting is a particular simple process to make ceramic components, in particular preforms as basis for MMC components. Slip casting is in particular suitable for the production of preforms comprised of SiC, although preforms made of different ceramic powders such as carbides, nitrides, borides, oxides or mixtures thereof such as, oxynitrides, can be made by slip casting as well. Concrete materials for the ceramic powder include, for example, SiC, TiC, B4C, AlN, Si3N4, BN, and Al2O3.
In the description, the term xe2x80x9cslipxe2x80x9d will denote a mixture of a powder with a particular amount of liquid carrier. In this context, the slip includes a ceramic powder to which, for example, water as liquid carrier has been added. When mixing liquid carrier to a ceramic powder, the flow limit of this dispersion is decreased to such an extent as to obtain an intrinsically viscous liquid. This dispersion behaves completely rigidly below a particular tension while being deformable without any resistance above this particular tension. Application of shearing forces decrease the flow limit, and the slip can be cast to the desired configuration.
A further increase of the fraction of liquid carrier results in a fluid slip, i.e., the slip can be cast in a mold without influence of shearing forces.
Conventionally, the production of a ceramic structure by means of a slip casting process, regardless whether the slip is fluid or has thixotropic properties, has been carried out by one of two options:
a) the slip contains, apart from the liquid carrier, no binders, and is introduced in two casting molds having porous walls.
b) the slip contains, apart from the liquid carrier, a binder, and is poured into casting molds of substantially dense consistency.
In the variation a), the porous casting molds are made predominantly of plaster, although ceramic and metallic porous materials have been considered as well. The porosity of the casting molds is intended to extract the liquid carrier from the introduced slip and to thereby slightly compact the slip. The extraction of the liquid carrier commences in areas of the slip that immediately adjoin the casting mold. Thus, as initially the outer layers of the resulting ceramic structure consolidate, the transport of liquid, still retained within into the pores of the casting mold becomes impaired, so that the extraction of liquid, realized by the porous casting mold, results in a poor compactness of the obtained ceramic structure. As a consequence, a further drying step is required to impart the ceramic structure with a sufficient stability that allows a removal or other manipulations. Moreover, the extraction of liquid is very slow as a consequence of the capillary effect of the pores of the casting mold so that the overall production takes too long to be considered economical. During this extraction of liquid, the fine particles of the ceramic powder are entrained with the extracted liquid to a greater extent than the coarse particles and collect primarily in the bottom area of the casting mold. Thus, an undesired concentration gradient is experienced in the final ceramic product. Moreover, the fine particles may also deposit in the porous casting mold, thereby effecting a certain bond of the resulting ceramic structure with the casting mold so that the detachment of the ceramic structure from the casting mold becomes significantly more difficult. A further drawback is the very slight density of the resulting ceramic structure, i.e. the ceramic structure has a relatively great porosity. In order to provide this ceramic structure with a denser consistency, sintering is required, resulting, however, in a shrinkage so that the dimensions of the structure are reduced to an extent that is not negligible. The manufacture of precisely predefined dimensions of the ceramic structure is thus very difficult to realize with this type of method.
In the variation b), no extraction of liquid carrier is realized so long as the slip remains in the casting mold. Rather, consolidation of the slip is implemented by freezing the liquid carrier. Subsequently, the slip is removed from the casting mold, dried through sublimation or evaporation, and sintered. Also this method suffers shortcomings. The resultant structure has also only a relative low density. Further, the liquid carrier not only freezes within itself and holds the particles of the ceramic powder tightly together but freezes also upon the wall surface of the casting mold so that the detachment of the ceramic structure from the casting mold causes problems. A change of material of the casting mold and/or using different carrier liquids is unlikely to overcome this problem. The use of casting mold surfaces which have been polished in a particularly smooth manner may lead to better results, however it is complicated to fabricate. The application of release agents on the casting mold surface may be conceivable; However, there is the risk of introducing foreign matters into the ceramic structure.
Heretofore, the infiltration process, i.e. the introduction of metal into the pores of the preform, is carried out after the production of a preform in a manner described above, whereby the preform is placed into another casting mold that is different than the casting mold used for producing the preform. Thus, the MMC manufacturing process, comprised of preform production and infiltration with metal, has the problems associated with the transfer of the preform from the first casting mold to the second casting. These problems are primarily based on the difficulty to detach the preform from the first casting mold as well as on the necessity to provide the preform with a sufficient strength to permit further processing, through provision of a further method step (freezing or sintering of the slip) while the slip is in the first casting mold.
It is thus an object of the present invention to provide an improved process for making MMC components, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved process for making MMC components, which is simple and faster to carry out and yet reliable.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by mixing particulate ceramic powder with a liquid carrier, without addition of a binder, to prepare a slip having thixotropic properties; introducing the slip into a casting mold of substantially dense consistence; subjecting the casting mold to vibrations so as to separate the carrier from the ceramic particles and to allow the carrier to float upon the ceramic particles while at the same time compacting the slip to realize a ceramic preform of porous consistency having pores; terminating the exposure of the casting mold to vibrations; removing the liquid carrier; allowing the preform to solidify in the casting mold, without exposure to any further compaction measures, such as sintering, pressing or the like, and maintaining the casting mold in a state of rest; and pouring matrix metal into the casting mold to fill the pores of the preform.
The process of making a metal matrix composite, in accordance with the present invention, is divided in substantially two stages, with the first stage directed to the manufacture of the porous preform, and the second stage directed to the subsequent infiltration of the preform with matrix metal, i.e. the matrix metal infiltrates the pores of the preform, whereby an essential feature of the present invention resides in the linkage of both stages with one another and the modification of measures between both stages.
As the preform is made in a casting mold in which it remains for subsequent infiltration, a removal of the preform from the casting mold before infiltration is no longer required so that the drawbacks associated with prior art processes have been eliminated. The preform remains in the same casting mold throughout, and thus no measure must be undertaken to impart a particular strength, thereby further eliminating a step required to date. The elimination of two method steps results in a significant simplification and acceleration of the MMC production.
Extraction of the liquid carrier from the slip through vibrating the casting mold is significantly faster to implement than conventional methods for extraction of liquid through heating or evacuation of the casting mold environment or through using porous casting molds.
Suitably, the slip is caused to vibrate before and during introduction thereof into the casting mold. The use of tools, such as ladles or scoops, separate from the slip container, for introducing the slip into the casting mold can thereby be omitted. Pouring of the slip into the casting mold may be realized by tilting the slip container, or by opening a valve in a connection conduit between the slip container and the casting mold.
According to another feature of the present invention, the extraction of the liquid carrier may be realized through heating the carrier to a temperature above the evaporation point of the liquid carrier. As the following infiltration of the preform with metal necessarily requires heating up the preform, respective kilns are anyway provided and can be used to implement the evaporation of the liquid carrier. Separate devices for extraction of the carrier, such as suction units are unnecessary, thereby further contributing to the simplification of the MMC manufacturing process according to the present invention.
Water, tertiary butanol, butanol-2 and amyl alcohol may be used as preferred substances for the liquid carrier. In particular water is cost-efficient and provides thixotropic properties of the slip in a majority of ceramic materials that are possible for formation of the preform. Non-aqueous carriers are not as cost-efficient, but their extraction can be carried out significantly faster and more complete.
Suitably, a ceramic powder with multimodal particle distribution is used. These types of ceramic powder results in a preform body, when subject to vibrations, with a particularly high packing density and higher strength, compared to a ceramic powder with monomodal particle distribution. It is also possible to use a ceramic powder having particles made of ceramic carbides, nitrides, borides, and oxides, such as, e.g. SiC, TiC, B4C, AlN, Si3N4, BN, and Al2O3. These materials exhibit different mechanical, electrical and thermal properties, but are all suitable for use in the process according to the invention. A suitable selection of the used ceramic powder that may also be a mixture of different ceramic materials, permits an adjustment with respect to the mechanical, electrical and thermal properties of the resulting preform and of the ultimately resulting MMC body. Preforms containing SiC partially or entirely, lead to the formation of MMC components that exhibit especially good heat conductivity as well as especially high mechanical strength.
Examples for matrix metal include Al, Ni, Co, Fe, Mo, Mn, and Cu. Also these materials have different mechanical, electrical and thermal properties, but are all suitable for use in the process according to the invention. Through appropriate selection of the infiltration metal, which may also be an alloy of various pure metals, the mechanical, electrical and thermal properties of the resultant MMC body can be adjusted.
According to another feature of the present invention, a porous ceramic body may be placed upon the bottom of the casting mold before introducing the slip into the casting mold whereby the porous ceramic body has a porosity which is greater than the porosity of the preform obtained from the slip. Suitably the porous ceramic body has the configuration of a board of slight thickness. Some applications require to provide the MMC component with a particular flexure or to precisely adjust the evenness of the MMC plate. This can be implemented through placement of a highly porous ceramic board into the casting mold. As a consequence, a preform is made having two zones of different porosity: the zone of the inserted board has a greater porosity than the zone of slip above. During infiltration of this preform with metal, more metal can penetrate into the bottom zone of greater porosity of the resulting MMC component compared to the top zone so that the MMC component is provided with two layers of different structure and different properties. The bottom layer with the significantly higher share on metal causes the entire MMC component to bend in its direction (same effect as in a bimetal). Through appropriate selection of the thickness of the inserted ceramic body, the degree of flexure can be adjusted.
In this context, it is advantageous to use a ceramic body made of the material of the ceramic particles contained in the slip, because in this case, the only difference between both zones of the resulting MMC component resides in the amount of infiltration metal so that the differences with respect to properties of both these zones can fairly easily be assessed. Suitably, the porous ceramic body may be made by introducing into the casting mold, before introduction of the slip, an emulsion comprised of fine ceramic powder with a mean particle size of less than 15 xcexcm, or a powder mixture 70/30 with a particle size of 12 xcexcm and 3 xcexcm, and a liquid carrier, and then smoothing and drying the emulsion. Thus, the formation of porous ceramic bodies can be executed with the same tools (casting mold, vessels, etc.) as used for producing the second preform zone from the slip, so that the extra time required for making the porous ceramic body in this embodiment of the process according to the invention is essentially negligible. Hereby, it is advantageous to use a ceramic powder which is made from the material of the ceramic particles contained in the slip in order to realize the afore-mentioned formation of the ceramic body from the material of the other preform zone.
According to another feature of the present invention, ceramic particles may be sucked off, before the pouring step, from a surface area of the preform to produce structures, such as bores and grooves. In this manner, the preform can be manipulated without exposure to significant wear. Therefore, the technical expenditure is significantly reduced as a result of the reduced wear since the use of high-strength tools, as required to date in conventional processes after a sintering process, is no longer required. Suitably, before removing ceramic particles, the respective surface area is loosened by means of hobs or dies or the like. The material being removed can then be withdrawn at significantly less suction powder, so that the risk of inadvertently sucking away ceramic particles from neighboring areas is significantly reduced and structures with particularly precisely defined contours can be realized. Optionally, it is possible to insert locators or inserts in the structures formed through removal of ceramic particles from certain surface areas. These locators prevent a penetration of infiltration metal into the produced structures and after removal of these locatorsxe2x80x94which is easier to accomplish than a removal of infiltration metalxe2x80x94the structures in the final MMC component are created.